This invention relates to a composite fabric for use as clothing for the sheet forming section of a papermaking machine and, in particular, to a composite fabric comprising at least two fabric layers interconnected by binder threads
Clothing for the sheet forming section of a papermaking machine, so-called sheet forming screens or papermachine screens, should have a smooth top side (paper side) in order to avoid any marks in the paper. On the other hand, the bottom side (backing side) has to be formed so as to impart to the sheet forming screen a long service life. This is required since the use of less expensive and more abrasive filler materials and the increase in operating speed subject the backing side to high wear.
Even in single-layer papermachine screens, the two fabric sides of most types of fabric are different. Thus, the paper side comprised predominantly of warp and weft threads interwoven in monoplanar fashion is smoother and the backing side comprised of weft wire knuckles in the cross fabric direction (weft runners) is rougher.
In the case of double-layer papermachine screens, this difference in the character of the two fabric faces or sides is even more pronounced. With this type of screen, the warp threads are common to both fabric sides. The weft threads, in turn, are divided into two separate weft layers and can be adapted to the requirements of the respective screen surface as regards the material and the thread diameter Morever, each side can be given any desired surface structure independently of that of the other screen side.
However, complete separation of the two screen sides is possible only with so-called two-layer screens. These screens comprise two completely independent fabric layers interconnected by an extra binder thread. Screens of this construction are known from German Offenlegungsschrift Nos. 2,455,184 and 2,455,185. In particular, these references teach circularly woven screens with a binder warp. This implies that in the final screen the two layers are interconnected by transversely extending binder threads.
Interconnection of the two fabric layers by a binder warp, however, has the drawback that during weaving the warp is under tension (weaving tension) so that it influences the structure on the paper side. Furthermore, when a two-layer fabric with a binder warp is woven flat and is made endless by means of a woven seam, the binder warp in the final screen extends in the longitudinal direction. Since the fabric is lengthened during thermosetting in the heating zone, the warp threads are again subject to high working tension. Owing to the fact that the weft threads of the lower layer are substantially thicker and stiffer, the tension of the binder warp affects nearly exclusively the finer threads of the upper layer. Thus, the binder warp pulls the fine weft threads of the upper layer deep into the fabric at the binding points thereby causing non-uniformity in the surface.
The above shortcoming can be remedied to a certain extent by interconnecting the two layers with a binder weft as described in German OS 2,917,694. Although ultimately the two types of fabric are identical--in both fabrics the two layers are interconnected by the additional transverse threads--the manufacture is somewhat easier because in a flat woven and seamed screen, for example, the two layers are interconnected during weaving and during setting by means of a transverse thread (weft thread). However, even when this measure is taken a uniform surface structure of the top layer is not produced, because at the binding points the additionally interwoven binder weft pulls the upper warp deep into the fabric thereby causing undesired depressions at the binding points in the fabric surface.
More particularly, the binder weft thread is placed under tension during weaving when the binder thread, which is initially inserted straight by the shuttle, is crimped upon the change of the harness frame position. The crimped binder weft extends in zig-zag fashion alternately between the upper and lower layers of the composite fabric which are relatively widely spaced apart. Owing to this longer path, the binder thread is already placed in a stretched condition during weaving. Since the lower layer comprises relatively thick, unyielding warp and weft threads, all the tension of the binder weft thread in this case, too, is transmitted to the binding points in the upper layer, because it is solely the structure of the upper layer that is able to yield. This results in a change in the structure of the upper layer at each binding point during the weaving operation.
Furthermore, during heat-setting there is crimp interchange between the warp and the weft wires of the two layers. The warp of the lower layer is stretched and its knuckles are flattened. The space between the lower binding points and the upper fabric layer is enlarged. Since the lower warp is stiff and unyielding, the upper layer is pulled even deeper into the fabric at the binding points.
The influence of temperature during setting releases shrinkage forces inherent in the binder weft thread. These forces act as an additional tensile force affecting the thin upper warp at the binding points and contributing to the non-uniformity of the surface structure.
During the manufcture of some paper types the non-uniformity of the surface at the binding points of the upper screen are of no consequence. However, in certain types of paper highly sensitive to screen marks--such as gravure printing papers, offset and imitation art papers--such sites result in printing imperfections which recur over the entire area of the paper web in uniform distribution corresponding to the weave pattern.
It is therefore a primary object of the present invention to provide a composite fabric for use as clothing for the sheet forming section of a papermaking machine which is comprised of at least two fabric layers interconnected by binder threads and which exhibits improved uniformity of the surface structure on the paper side.